1. Field of the Invention
The present invention relates to an optical pickup and an optical disc drive supporting a plurality of optical disc standards.
2. Description of the Related Art
Optical discs for optically recording and reproducing information are classified into three types, for which laser beams with different center wavelengths are used. The center wavelength is 0.78 micrometer (0.78 μm) for a compact disc (CD), 0.65 μm for a digital versatile disc (DVD), and 0.405 μm for a high-definition DVD (HD-DVD) or a Blu-ray disc (BD). HD-DVDs and BDs are both referred to below as blue optical discs. To provide compatibility with all three types of optical discs, recording and reproducing devices for blue optical discs also need to support the DVD and CD standards. In addition, double-capacity dual-layer discs having two signal layers are available under the DVD and blue optical disc standards, and further research and development that will increase the number of signal layers is currently in progress (see, for example, Ichimura et al., “8-so 200-gigabaito bururei disuku (Eight-layer 200-gigabyte Blu-ray Disc)” O PLUS E Vol. 27, 4th issue, April, 2005 published by New Technology Communications pp. 425 to 430, FIG. 9).
As semiconductor laser light sources, semiconductor laser devices capable of generating laser beams with two different wavelengths (0.65 μm and 0.78 μm) from a single chip or package are already commercially available. Semiconductor laser devices capable of generating laser beams with wavelengths of 0.405 μm as well as the above two wavelengths (0.65 μm and 0.78 μm) from a single package are also becoming commercially available (see, for example, Hashizu et al., “Aomurasaki-iro handotai reza (Bluish-purple Semiconductor Laser” O PLUS E Vol. 27, 4th issue, April, 2005 published by New Technology Communications pp. 339 to 404, FIGS. 1 and 12).
Turning to the objective lens for focusing the laser beams onto the information recording surface of the optical disc, a single objective lens capable of focusing laser beams with two different wavelengths is used in conventional DVD/CD-compatible optical disc drives. Objective lenses through which laser beams with three different wavelengths, including a blue wavelength, can be focused have also become available.
An optical pickup having a single objective lens and two semiconductor laser packages for generating laser beams of three different wavelengths, and an optical pickup having a single objective lens and a single semiconductor laser package for generating laser beams of three different wavelengths have been proposed (see, for example, Matsumiya et al., Japanese Patent Application Publication No. 2005-339771, pp. 7 and 13, FIGS. 1A, 1B, 2, and 3).
In the optical pickups described by Matsumiya et al., the optical detection system that detects the laser light reflected from the optical disc is also integrated into a single unit. Besides detecting the signal to be reproduced from the optical disc, this optical detection system simultaneously detects signals needed for focusing control and tracking control. Because the specifications of blue optical discs, DVD optical discs, and CD optical discs differ, however, it is extremely difficult to detect the signals required by each of the optical disc specifications with a single optical detection system. In particular, detection of the focus error signal needed for focusing control with a single optical detection system poses problems.
One problem is that a single optical detection system has a single linear range. The linear range is the range over which there is a linear relation between the focus error of the focused laser beam and the focus error signal obtained from the output of the photodetector. With a single linear range, it is difficult to design a control system that provides optimum focus control for three or more types of optical discs.
To explain this problem in more detail, the depth of focus calculated from the wavelength of the light source and the numerical aperture of the objective lens is used as an estimate of the focusing tolerance in focusing control. If the Blu-ray Disc standard (BD standard), which has the highest density among the above standards, is compared with the CD standard, which has the lowest density, the depth of focus in the BD standard is only approximately 1/7 of the depth in the CD standard. The DVD and blue DVD standards standard also include specifications for commercially available dual-layer discs with signal surfaces one above the other, and the research and development work mentioned above has produced announcements from several organizations of the development of discs with up to sixteen layers, so it is necessary to take account of the future appearance of multilayer discs with three layers or more. To perform focus control with a clear distinction between the individual layers of such multilayer discs, the linear range must be kept sufficiently less than the spacing between adjacent pairs of layers. If the linear range is made that small, however, the problem of extremely demanding focusing control for a CD arises, because in comparison with the other types of optical discs, a CD has a larger allowable wobble of the optical disc surface in the focusing direction. If the depth of focus is made large enough to accommodate the allowable CD wobble, however, then accurate focusing control for the higher-density optical discs becomes problematic, and distinguishing between the multiple layers of a multilayer disc becomes impossible. It is also necessary to take account of the inter-layer spacing in a multilayer disc. The inter-layer spacing in the dual-layer discs already commercialized has a center value of 55 μm for DVD discs and 25 μm for BD discs. The number of layers in future commercial discs and the spacing of the layers have not been decided yet, but it is reasonable to expect that the spacing will depend on the number of layers. The spacing between different layers may also be varied intentionally to mitigate the problem of stray light reflected from unintended layers, as proposed by Ichimura et al. In this case, the linear range will have to be optimized not only for the type of disc being used but also for the particular layer being accessed in the disc. The approximately factor-of-seven difference between the depths of focus required by the BD standard and CD standard makes it impossible to set a single linear range that can simultaneously meet the needs of both standards. With a conventional single optical detection system, it is not possible to create an optimal focus error signal for the blue disc standards, the DVD standard, and the CD standard. For a disc with three layers or more, it will not be possible to provide an optimal focus error detection unless the linear range can be adjusted to suit the number of layers and their spacing. Moreover, the linear range is also affected by manufacturing error in the focal length of the lenses in the optical pickup, positioning error in the mounting of the lenses and the assembly of the pickup, and variability in the lasing wavelength of the laser light sources, so even for the same type of disc, the linear range of the optical pickup varies from one manufactured pickup to another. For discs with a large number of layers with exacting spacing requirements, the linear range needs to be precisely defined. A conventional single optical detection system is incapable of providing the active control that would be needed to adjust the linear range to the different values required by multilayer discs or different layers in such discs.